Glass substrates which are used for a display application are generally formed according to a float method, a down draw method represented by an overflow down draw method, or the like.
The float method is a method of casting a molten glass onto molten tin (float bath) and stretching it in the horizontal direction to form the glass in a sheet form. According to the method, a glass ribbon is formed on the float bath, and the glass ribbon is then annealed (on-line annealed) in a long annealing furnace. Accordingly, the glass substrate formed according to the float method is characterized by having a small thermal shrinkage ratio.
However, the float method involves such disadvantages that it is difficult to make the sheet thin, the glass substrate is required to be polished to remove tin attached onto the glass surface, and the surface quality of the substrate is lowered.
On the other hand, the down draw method is a generic term for a forming method of drawing a glass in the vertical downward direction to form it in a sheet form, and a slot (slit) down draw method, an overflow down draw method, and the like are known. For example, in the overflow down draw method that is widely adopted, a molten glass is introduced into the top of a trough-shaped refractory (forming body) having a nearly wedge-shaped cross section, and the glass is allowed to overflow out from the both side thereof to flow down along the side face, and the two streams are joined together at the lower end of the refractory and drawn downward to form the glass in a sheet form. The down draw method is advantageous in that a glass is easy to be formed into a thin sheet.
Furthermore, in the case of the overflow down draw method, since the glass surface does not come into contact with any other than air, there is also such an advantage that a glass substrate having high surface quality can be obtained even in an unpolished state.